Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a NullPointerException

Fixes https://issues.oss-fuzz.com/issues/42537550

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1923060 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xdgf/usermodel/section/geometry/EllipticalArcTo.java

@@ -73,6 +73,10 @@ public class EllipticalArcTo implements GeometryRow {
         for (CellType cell : row.getCellArray()) {
             String cellName = cell.getN();
 
+            if (cellName == null) {
+                throw new POIXMLException("Missing cellName in EllipticalArcTo row");
+            }
+
             switch (cellName) {
                 case "X":
                     x = XDGFCell.parseDoubleValue(cell);